Integrated actuator meniscus mirror

ABSTRACT

An integrated actuator meniscus mirror includes an optical substrate having a mirror surface on one side and a support structure on the other and a plurality of actuators embedded in the support structure, spaced from and generally parallel to the mirror surface for applying bending moments to the mirror surface for controllably altering the shape of the mirror surface.

RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional PatentApplication No. 60/450,198 filed Feb. 25, 2003, entitled “AN INTEGRATEDACTUATOR MENISCUS MIRROR”.

FIELD OF THE INVENTION

[0002] This invention relates to an integrated actuator meniscus mirror.

BACKGROUND OF THE INVENTION

[0003] Meniscus mirrors typically employ an optical substrate containingthe mirror surface, a reaction mass, and a number of actuators foraltering the shape of the mirror to a desired excursion (global orradius of curvature shaping of the entire mirror) or to effectcorrectability (local altering of the shape to overcome distortion) frome.g., heat, vibration, gravity. The mirrors may be operated zonally;each zone of the mirror has a local sensor which controls a localactuator to shape that area of the mirror or modally: the entire mirrorsurface is sensed globally (e.g. using an interference pattern) and thenthe local actuators are operated to effect the sloping. The actuatorsare generally oriented normal to the mirror surface and require thereaction mass to exert the proper deformation to the mirror to overcomethe distortion. One problem has been that the need to lighten the weightof the mirror, exacerbated by the presence of the reaction mass, hasresulted in lighter and thinner mirrors to the point where they areextremely fragile and difficult to fabricate and process. Anotherproblem is the varying performance of the different types of actuators.Surface normal actuators (SNA) of the displacement type afford goodcorrectability but poorer excursion with high natural frequency and highareal density. SNA's of the force type are moderately good atcorrectability and high excursion, have lower level natural frequencyand a moderate areal density. So called edge or radius of curvatureactuators which generally just bend the mirror globally have highexcursion but low correctability, a moderate natural frequency and lowareal density. SNA's of the displacement type can be smaller and so canbe packed more densely but they have limited capability displacementwhich places them at a disadvantage for effecting excursion or radius ofcurvature adjustments. SNA's of the force type have a betterdisplacement range but cannot be packed too densely and so they cannoteffect the best correctability. The edge or radius of curvatureactuators simply bend the mirror about its center using a limited numberof edge actuators and so are limited in their application forcorrectability. One shortcoming of all of these approaches is that theyrequire a reaction mass which increases the size and weight of themirror. Another is that the actuators are not easily installed orreplaced. Further, since the optical substrate is typically glass orberyllium and the reaction mass is graphite composite there is a thermalmismatch which introduces its own distortion.

BRIEF SUMMARY OF THE INVENTION

[0004] It is therefore an object of this invention to provide animproved integrated actuator meniscus mirror.

[0005] It is a further object of this invention to provide such animproved integrated actuator meniscus mirror which requires no reactionmass.

[0006] It is a further object of this invention to provide such animproved integrated actuator meniscus mirror which performs well forboth correctability and excursion.

[0007] It is a further object of this invention to provide such animproved integrated actuator meniscus mirror which has a higher naturalfrequency and lower areal density.

[0008] It is a further object of this invention to provide such animproved integrated actuator meniscus mirror whose actuators areembedded in the optical substrate of the mirror not only obviating thenecessity for a reaction mass but making installation and replacement ofthe actuators much simpler.

[0009] It is a further object of this invention to provide such animproved integrated actuator meniscus mirror which enables addedmaterial to be used in the optical substrate of the mirror where it canimprove its strength and rigidity and its suitability for manufacturingsince the weight of the reaction mass has been eliminated.

[0010] This invention results from the realization that a trulyimproved, light weight, integrated actuator meniscus mirror requiring noreaction mass yet having good natural frequency, areal density,excursion, and correctability characteristics can be achieved with anoptical substrate including a mirror surface on one side and a supportstructure on the other and a plurality of actuators embedded in thesupport structure spaced from and generally parallel to the mirrorsurface for controllably altering the shape of the mirror surfacelocally and globally, zonally or modally and further to do so withoutthe need for a reaction mass.

[0011] This invention features an integrated actuator meniscus mirrorincluding an optical substrate having a mirror surface on one side and asupport structure on the other. A plurality of actuators are embedded inthe support structure spaced from and generally parallel to the mirrorsurface for applying bending moments to the mirror surface forcontrollably altering the shape of the mirror surface.

[0012] In a preferred embodiment the optical substrate may includesilicon carbide. The support structure may include an array ofintersecting major ribs. Each actuator may be mounted in a major ribbetween the intersections. The support structure may include cathedralribs on the back side of the mirror surface. Each rib may contain arecess for receiving an actuator. An actuator may include anelectrostrictive device. Each actuator may include a lead-magnesiumniobate electrostrictive device. The support structure may include anarray of spaced posts and each actuator may extend between a pair ofspaced posts. The bending movements are developed without resort to areaction mass. The substrate may be formed of any type of opticalmaterial and the actuators may be any type of voltage controlledactuators.

BRIEF DESCRIPTION OF THE DRAWINGS

[0013] Other objects, features and advantages will occur to thoseskilled in the art from the following description of a preferredembodiment and the accompanying drawings, in which:

[0014]FIG. 1 is a three dimensional diagrammatic view of the mirrorsurface of the optical substrate of the integrated actuator meniscusmirror according to this invention;

[0015]FIG. 2 is a three dimensional view of the other side of theoptical substrate of FIG. 1 showing the support structure and undersideof the mirror surface;

[0016]FIG. 3 is an enlarged three dimensional view of a portion of thesupport structure of FIG. 2 with actuators installed;

[0017]FIG. 4 is an enlarged three dimensional view of an actuator andactuator mounting;

[0018]FIG. 5 is an enlarged three dimensional view of another actuatorand actuator mounting implementation;

[0019]FIGS. 6, 7, and 8 are graphs illustrating the factors effectingstiffness, excursion and correctability, respectively;

[0020]FIG. 9 is a three dimensional view of another support structureaccording to this invention; and

[0021]FIG. 10 is a diagram showing the method embodied in software in amicroprocessor for driving the actuator to manipulate the shape of themirror.

DISCLOSURE OF THE PREFERRED EMBODIMENT

[0022] Aside from the preferred embodiment or embodiments disclosedbelow, this invention is capable of other embodiments and of beingpracticed or being carried out in various ways. Thus, it is to beunderstood that the invention is not limited in its application to thedetails of construction and the arrangements of components set forth inthe following description or illustrated in the drawings.

[0023] This invention features integrated actuator meniscus mirror 10,FIG. 1, including an optical substrate 12, typically silicon carbide oran equivalent, including an optical such as metal glass, ceramic,polymer and components thereof including but not limited to a FusedSilica, ULE, Zerodur, Al 6061-T6, MMC 30% SiC, Be I-70. Be I-220-H, CuOFC, Cu Glidcop, Invar 36, Super Invar, Molybdenum, Silicon, SiC HPalpha, SiC CVD beta SoC RB 30% Si, C/SiC, SS 304, SS 416, SS 17-4PH, Ti6A14V, Gr/EP GY70x30, having mirror surface 14 on one side and supportstructure 16, FIG. 2, on the other side. The support structure mayinclude a plurality of major ribs 18, which intersect at a node 20 atthe center of a zone of influence. Each major rib, such as rib 18 a,includes recess or notch 22 in which an actuator may be located. Thearray of major ribs creates a honeycomb-like structure supporting backside 24 of the mirror surface on which can be located cathedral ribs 26for strengthening and further supporting mirror surface 14. The sixholes 28 which coincided with particular nodes 20 are not germane tothis invention but are used to receive three pairs of bipods whichconnect to a mounting plate and form a part of the metering structurethat supports the primary and secondary mirrors and additional equipmentwhich, for example, make up a telescope or beam director.

[0024] Actuators 30, FIG. 3, are embedded in recesses 22 of ribs 18generally parallel to the mirror surface and spaced from it. Whenoperated either by extension or contraction, actuators 30 apply bendingmoments to alter the shape of the mirror, both locally forcorrectability, and globally to effect radius of curvature alterations.Because actuators 30 act directly on the support structure in which theyare embedded, they require no reaction mass. In addition, even thoughthey may be displacement devices, they can perform a very effectiveradius of curvature or excursion shape alteration because their effectis cumulative.

[0025] Each of the actuators 30 may be an electrostrictive device or amagnetostrictive device, a piezoelectric device or any other suitabletype of actuator such as hydraulic, voice coil, solenoid, mechanical orphase change material such as shape memory alloys or paraffin. In thispreferred embodiment, they are illustrated as electrostrictive devicesof the lead-magnesium niobate or PMN type which are preferred becausethey have a low thermal coefficient and very little hysteresis and creepand are dimensionally stable to sub-Angstrom levels. The actuators arecharacteristically easy to install and replace. For example, actuator 30a, FIG. 4, may contain mounting tabs 32 and 34 which are receivable inmounting clips 36 and 38 mounted in notch 22 b of rib 18 b. Slots 36 and38 may be mounted to rib 18 b by means of clamps 40 and 42. All of theinterfaces may be supplied with an adhesive to permanently bond actuator30 a in position. The actuators may be ambient temperature actuators orcryogenic actuators so that the mirror can be converted from one type ofoperation to another quite easily by simply removing one type andreplacing it with the other.

[0026] Another type of actuator mounting is shown in FIG. 5 where athree step installation is shown beginning with the actuator 30 c beingsupplied with bonding tabs 32 c and 34 c which may be glued to it. Thisassembly is then installed in recess 22 c of major rib 18 c by engagingthe slots 40 and 42 in tabs 32 c and 34 c with the edges of recess 22 cso that the final assembly appears as at 50 in FIG. 5. Again, some orall of the engagements may have an adhesive applied to bond thecomponents.

[0027] The efficacy of this invention is illustrated in FIGS. 6, 7, and8. In FIG. 6 the trade-offs with respect to stiffness are displayedwhere it can be seen that for a design window 52, FIG. 6, defining anareal density of 10 kg/m² or less, a high stiffness of 1.0E+06 inchpounds can be achieved in conjunction with that low areal density whilemaintaining a fairly high 300 Hz natural frequency. FIG. 7 illustratesthe trade-offs with respect to excursion where the surface deformationassociated with excursion and gravity sag are both in satisfactoryranges expressed in sectional stiffness in inch pounds. The trade-offwith respect to correctability is demonstrated in FIG. 8 where thecorrectability is plotted against Zernike polynomials indicating thatthe localized correction or correctability performs quite well even athigh Zernike polynomials with adequate numbers of actuators. Andadequate numbers of actuators is not a problem as they are small,lightweight, and can be highly densely packed.

[0028] Although the support structure shown is a honeycomb likestructure formed from the intersecting ribs, this is not a necessarylimitation of the invention as any structure which enables the actuatorsto be spaced from and generally parallel to the mirror surface may beused. For example, in FIG. 9 the support structure on back surface 24 aof the mirror constitutes spaced bumps or dimples or posts 60 and theactuators 18 d are connected between pairs of posts effecting thebending moments and creating the nodes as previously explained withrespect to the honeycomb structure.

[0029] Any suitable hardware or software system may be used to monitorand feedback control signals to the integrated actuator meniscus mirroraccording to this invention. One such system is illustrated in FIG. 10by way of example and not limitation. There microprocessor 70 drives I/Odevice 72 to provide voltages to actuators 18′. The zygo image 74 isgenerated from mirror surface 14, FIG. 10. Microprocessor 70 isconfigured with software to establish a reference FIG. 76 and thenestablish for each actuator an influence function on its associatednodes or zones 78. The mirror is then exposed to a distortingenvironment 80 and once again measured in step 82. The reference is thensubtracted from the measurement to get residual error 84 and theresidual error is decomposed 86 into actuator commands which are thenapplied to actuator 88 through I/O device 72 to provide the propervoltages to actuators 18′. This routine is carried out repeatedly inorder to keep the mirror at the optimum shape. Although the preferredembodiment discussed above is generally of the zonal type the integratedactuator meniscus mirror of this inventor may be implemented as a modeltype or any other type.

[0030] Although specific features of the invention are shown in somedrawings and not in others, this is for convenience only as each featuremay be combined with any or all of the other features in accordance withthe invention. The words “including”, “comprising”, “having”, and “with”as used herein are to be interpreted broadly and comprehensively and arenot limited to any physical interconnection. Moreover, any embodimentsdisclosed in the subject application are not to be taken as the onlypossible embodiments.

[0031] Other embodiments will occur to those skilled in the art and arewithin the following claims:

What is claimed is:
 1. An integrated actuator meniscus mirrorcomprising: an optical substrate including a mirror surface on one sideand a support structure on the other; and a plurality of actuatorsembedded in said support structure spaced from and generally parallel tosaid mirror surface for applying bending moments to said mirror surfacefor controllably altering the shape of said mirror surface.
 2. Theintegrated actuator meniscus mirror of claim 1 in which said opticalsubstrate includes an optical material.
 3. The integrated actuatormeniscus mirror of claim 1 in which said optical substrate includes anoptical material from the group metals, glasses, ceramics, polymers andcomposites thereof.
 4. The integrated actuator meniscus mirror of claim1 in which said optical substrate includes silicon carbide.
 5. Theintegrated actuator meniscus mirror of claim 1 in which said supportstructure includes an array of intersecting major ribs.
 6. Theintegrated actuator meniscus mirror of claim 5 in which each saidactuator is mounted in a said major rib between said intersections. 7.The integrated actuator meniscus mirror of claim 1 in which said supportstructure includes cathedral ribs on the back side of said mirrorsurface.
 8. The integrated actuator meniscus mirror of claim 6 in whicheach said rib contains a recess for receiving a said actuator.
 9. Theintegrated actuator meniscus mirror of claim 1 in which a said actuatorincludes an electrostrictive device.
 10. The integrated actuatormeniscus mirror of claim 9 in which said actuator includes a leadmagnesium niobate electrostrictive device.
 11. The integrated actuatormeniscus mirror of claim 1 in which said support structure includes anarray of spaced posts.
 12. The integrated actuator meniscus mirror ofclaim 11 in which each said actuator extends between a pair of saidspaced posts.
 13. An integrated actuator meniscus mirror comprising:surface for controllably altering the shape of said mirror surfacewithout a reaction mass.